And after a walk that was frankly too long, Twilight's group finally got to a place where they could view the entrance of the cave.
It wasn't just a hole in the ground, in fact it was far from it. The entrance of the cave was taken up by an arch built directly into the rock. It was a big arch, enough that ten ponies could confortably walk together into it side by side, and twelve pegasi could confortably fly into it forming a pillar, one above the other. The arch itself was a magnificent creation of silver and gold, detailed with intricate patterns that called boundaries to mind. Those patterns were done in a variety of styles, some of them engraved, some painted on, some even made by adding "nails" (that was what they looked like) in a pattern.
The areas of the cave around the arch also weren't bare, being painted with representations of important scenes in the history of the Amulet of Threshold that was kept in the cave, and those paintings were done in a variety of styles, by a variety of artists. Some of them were naturalistic and seemed almost like pictures that could have been taken at the time, but others were so stilized that even with knowledge of what the painting was representing (thankfully provided by plaques under each one of them), it was still difficult to understand exactly what you were looking at. (One of the paintings made Scootaloo stop in surprise, since it was a representation in comic book style sequential art of a particularly action packed event. The pegasus filly took a moment to wonder about the possibility of a painting cutie mark.)
But they could not simply keep admiring the outside of the cave. Eventually the group of tourists decided they had seen enough and slowly entered into the actual cave, still being escorted by the four guards.
And they saw that the interior of the cave was even more magnificent than the exterior. In fact, it was so amazing that even if none of the four tourists were actually there for 'touristing', they still couldn't help but be amazed by what they were seeing. That was a crystal cave, with the walls and ceiling being completely covered in the crystals. The crystals were unpolished, but their variety of sizes, shapes and colors created an incredible scene within the cave, with their light reflecting of each other and making beautiful reflexes that changed colors every time the viewers moved their heads.
And the natural beauty of the place was only one part of its charm. Since that cave was the location of the Amulet of Threshold, the town turned it into a museum of of magical artefacts. Obviously they couldn't get other magical artefacts just for the purpose of showing them off to tourists, but they had many replicas, pictures and scrolls about famous artefacts. This all could seem strange, considering this place was supposed to exist to keep the Amulet of Threshold out of the wrong hands, but this just showed how much the magical protections in the Amulet were trusted, that ponies were allowed to get so close to it on such a large scale.
And Twilight definitely didn't need to do a lot of "pretending" to be a tourist, considering how much magical artefacts were intertwined with stories of famous Legendary Heroes and Knights, which was exactly the kind of thing the three apprentices of Celestia had liked to read when they were younger (and in fact still liked to read). For example:
"Oh! That's a reproduction of one of the paintings made by the master Bernlion regarding the legendary battle of Knight Revolution against the Black Claw! It was said he got so close to the battle that he only escaped death by luck. The original is in one of the art galleries of the Griffin Republic, considering this is an important part of their story too.
"Ah! And that's a model of the Broken Catedral, the building sized artefact that the followers of the Shadow Ruler tried to use to force him into our dimension, only to be stopped by the Adventurers Six, a group including a minotaur, a seapony, a pegasus-dragon hybrid, a doe, an earth pony and a diamond dog. Definitely one of the strangest and broadest groups of heroes in history.
"And that's... A plaque mourning the Knights of the Scarlet Dream, a powerful group that was brought to an end by The Flames of Annihilation, one of their own members, and perhaps the most famous Fallen Knight in history." Twilight put a hoof in her heart and closed her eyes, somber.
But still, even with all of that, the chaotic unicorn didn't lose sight of their real goal, and soon she got in sight of it.
It was an opening into a corridor connected with the main cave. That opening had its own arch which was pure silver, much smaller, but with more complicated engravings. There were images of the Amulet of Threshold in several different styles adorning the walls around the arch. Right outside of this corridor was the closest any tourist ever got to the amulet, and the reason for that was obvious enough to be its own attraction.
"So... The invisible barrier that prevents ponies from getting close starts here?" Scootaloo asked, getting close to the arch and extending a hoof forward into the opening.
She couldn't extend it completely because of the barrier. It wasn't like a solid wall, but seemed to push back with exactly the same amount of strength it was being pushed. The pegasus filly put more and more strength into the barrier, including firming her rear hooves and pushing with both front hooves, and even flapping her wings to try and get the greatest amount of impulse possible. But nothing worked.
That was the first magical defence of the Amulet of Threshold, there to gently discourage normal ponies from trying to get closer. It was fairly casual, and far from heavy-duty, but the amount of power needed to break through it was more than what any normal pony could casually use. And if you actually managed to get through it, that just meant you were going to face the serious defences.
Of course, neither Rainbow Dash nor Pinkie Pie were normal ponies, but they obviously didn't want to break the barrier, so the two mares restricted themselves a lot as they played with it. Pinkie Pie doing a running start and jumping on the barrier, only to have all her momentum negated in an instant and Rainbow Dash pressing on every point of the barrier by using her flight to get there.
Twilight seemed to be only watching the others have fun, but in fact she was doing her mission, and the whole reason why the other three were playing with the barrier was to give Twilight a reason to just remain close to the barrier without being suspicious. They couldn't just keep doing this indefinitely, but Twilight Sparkle was good enough to be able to check everything before the guards started getting suspicious about the amount of time the tourists stayed there, even though she had to be careful to prevent any of them from detecting her use of magic.
The chaotic Knight carefully passed her magic through the magical formation that kept the defences powered like she was using a brush. It was a very delicate and complicated process, considering the sheer complexity of the spells she was analyzing and the various traps that were put there to prevent exactly the kind information gathering she was attempting, which were difficult to avoid even though she knew about them more or less, but Twilight enjoyed it anyway. She always felt like magic should be like this, precise, mysterious, powerful and dangerous.
Twilight Sparkle was looking for traces of magical tampering on the spell formations, to see if the changelings had already started trying to get through and how far were they in that case. It didn't take long before the unicorn scholar realized that yes, somepony had come through there and left some traces. But annoyingly, whoever did it was a particularly skilled mage, she kept going further and further and only finding the barest traces of foreign magic, not enough to understand what was the purpose of the mage who did it.
So she kept going deeper and deeper, to the point where she would have taken too long to do it, were it not for the fact that Twilight was pushing her mind to the limit and experiencing time at a fairly different rate from normal, enough to count every thousandth of a second, a state necessary for the working of the most dangerous and precise spells. She finally found a bigger concentration of the magical traces so she only needed to search-
Suddenly, she stepped on a tripwire.
Twilight's mind went on overdrive, her superior thought processes were disengaged and she made several quick jumps and connections even faster than she could have consciously thought of it. Her mind didn't put it all in a nice, easy to understand way, but what she discovered was something like:
Magical tripwire. Trap. Not defence formation. Foreign. Foreign mage. Waiting for intruders.
And in the ridiculously small period of time before the tripwire could actually activate and make the alert, Twilight slammed it with a burst of raw chaos magic, which confused the tripwire for long enough for her to weave a spell around it and stop whatever it was about to do, before forcefully returning it to an untripped state and erasing every trace of her passage she could find.
Then Twilight went back to the real world, she had to warn the others.
"Well, let's go." She said, the signal that she was done with her part.
It had only taken slightly less than ten seconds.
The rest of her group looked at her, and she used that moment to pass on a message to Rainbow Dash and Pinkie Pie via her glance.
(Twilight: Don't need to respond. The enemies wanted us here, they didn't try to break the defences, they wanted to lure ponies who would check it and be caught by the magical tripwire deviously hidden inside, so they would be alerted. I think they want to use us to get the Amulet.)
The two Squires froze almost imperceptly for a moment.
That really was something important. And alarming.
Applejack ran as fast as she could, only stopping once in a while to Look at things with her Eyes of Truth.
She was moving in a previously decided route to be able to check the biggest possible area as fast as possible, since they didn't know how much time it would take for her to get results, if it was even possible. The route also made it more difficult for pursuers to follow her, since it never left Applejack out in the open for long.
And all of those precautions worked, because before the pursuing guard could get close enough to see her, Applejack found the Changeling base.
Looking through the rock itself, she saw dozens of Changelings and hundreds of normal ponies trapped in cocoons. She searched for, and found, the hidden entrance of the base, and she memorized every single aspect about it that she could, just in case any of it became relevant.
But eventually, the guard did manage to reach her, and Applejack waited until he got close before talking.
"Ah, thank you! I didn't know where I was going!" She spoke in the most convincing manner she could.
"Okay." The guard said "You just need to follow..." He got stopped by the appearance of a group of four flying cloaked figures that were approaching fast.
Applejack turned to them, trying to figure out what the changelings were attempting, and she got her answer as the guard turned back to his true changeling form and blasted her in the back of the head with a spell.
The farmer mare could have used her aura defences to protect herself, but the highest priority plans didn't call for that. She let herself be knocked unconscious, but not before sending a message to Fluttershy via her feather. A message consisting of a single word.
Rarity was doing an important job. She had to determine which of the ponies in town were in fact disguised Changelings, she specially had to determine which of the guards had in fact been replaced by Changelings.
Applejack could have done this easily, but she was occupied with a more important mission, so Rarity had to use a fairly complex method to be able to identify the Changelings. It relied on the fact that, while the Changelings could actually change everything about themselves, including their own internal energy, doing so took far too much energy and concentration, so it wasn't done casually.
Even so, it was difficulty to test whether any given pony was a changeling or not, specially doing so in a way that wouldn't be harmful if the target wasn't actually a changeling. It was even harder to do that without the changeling perceiving what was being done.
But of course, there was a spell for that. And the reason why the changelings were still a major threat despite such a convenient spell existing was because the spell wasn't actually convenient in the least. It was based upon the principles of radar, generating a wave of magic that bounced on the internal magic of the target, returning information about what kind of magic it was.
As might be expected, doing such a thing was fairly noticeable, usually because the skin of the target was forced open to allow the passage of the magic, which was why casting spells directly on living beings was difficult. Doing such without causing harm and without allowing the target to feel what was happening took such a frightening amount of precision that, among the Knights, only Trixie had been able to master the spell. (She had bragged a lot to her partners when her apprentice managed the feat.)
The fact that Rarity could do so made her the perfect pony to execute this part of the plan, she was using the spell on every pony she could see (which was most ponies in town, due to their route) and surreptiously writing the details on a spell paper inside her knapsack (using telekinesis to write without seeing was far easier than what she was already doing), that spell paper was connected to a similar one which was being held by Captain Shining Armor, as well as copies for every member of the team, just in case.
In contrast with Rarity's success, Fluttershy wasn't really accomplishing much.
It wasn't unexpected. Trying to figure out plots from large-scale emotion sensing was pretty much purely theoretical stuff, and she was only able to distinguish certain trends, like feeling that something huge was being prepared... Which wasn't surprising at all. Likewise, talking with animals produced only information the Knights had already been able to deduce, such as weird gatherings of ponies who should have no connection, and ponies being kidnapped right out of their rooms.
The empathic pegasus was feeling pretty down, even though the others had warned her that this part of the plan was a long shot. However, she suddenly felt something that completely wiped those thoughts out of her mind. She felt an unforgettable emotional signature. She felt Venus.
For a moment, she doubted her own feelings, because that emotional signature wasn't radiating love like she had seen Venus do, but Fluttershy understood why that pegasus would want to be a little lower key inside the town.
The animal caretaker was thinking about possibilities of taking off to go after Venus, but that turned out to not be necessary, as their normal route took the group closer and closer to that signature, until they entered a cafe, where Fluttershy finally saw her goal.
It was a pegasus mare with a gray coat and blue mane, she her cutie mark was a cut tree branch and she wore glasses, but Fluttershy would not be fooled by that disguise. She quickly said "Venus" to her companions using eye language and slowly approached the mare, who was sitting on one of the tables.
"Hello, how are you?" She asked in a friendly manner, with a smile.
"I'm well, thank you." Venus responded politely.
Fluttershy approached a little more and whispered a few words "The Knights request your help."
Venus froze. That wasn't surprising, because the words Fluttershy had spoken were literally legendary, being part of quite a few old Knight stories. This was the way the Knights requested help from stranger ponies, and frequently it lead to the ponies in question making their mark in history as heroes. That was the first time those words had been spoken seriously in centuries. The Knights could have asked the help of their apprentices like this, but they weren't really "strangers" so this ended up being the first opportunity for that line to be used on their generation.
Fluttershy quickly took a feather off her wings and put it on the table, before whispering again "Keep that with you."
After that, she returned to her group and they continued on their usual route, leaving the cafe again and walking once more through the streets.
It didn't take long after that before Fluttershy received a message from Applejack, that message consisted of the word "Jackpot."
She stopped for a moment and caught the eye of Rarity and Sunset, before sending a message of "A5", the particular variation of the plan they had worked out for this situation.
The two nodded in understanding, and Rarity wrote A5 on the spell paper and put in an extra burst of magic to catch the attention of the ponies who had the papers.
Everypony was warned that the plan was going to start.